After installing Seasons I'm also having a mailbox glitch, though a different one. My sims can't get mail. Every time I use the 'Get Mail' option, they walk up to the mailbox, open it and then stop abruptly. Nothing happens and no items appear in sim's inventory (though there should be books from the book club and the ones my sim has written).
They can pay bills and donate to charity, but can't get new mail.